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| Home > Publications database > Rastertunnelmikroskopuntersuchung der Morphologie von Platinum(111) nach Ionenbeschuß |
| Book/Report | FZJ-2018-03714 |
1991
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/19102
Report No.: Juel-2569
Abstract: The temperature and fluence dependence of the morphology of Pt(111) after Ar'$^{+}$ and He$^{+}$ ion bombardment has been investigated systematically. For this purpose an ultra high vacuum system with a scanning tunneling microscope (STM) has been developed and built. It allows full sample temperature control down to 100K during all sample preparation and analysis steps. The discussion of the observed morphologies is preceded by an analysis of the conditions for STM-imaging of rough surfaces and by the presentation of a new and simple STM-based method for sputtering yield determination on virgin surfaces. One main result of the morphological analysis of the surface after Ar$^{+}$ ion bombardment is the confirmation of a transition from pit formation at lower temperatures to layer-by-layerremoval at higher temperatures. This transition around 700K becomes particularly obvious after large ion fluences : After the removal of 50 monolayers at 625K deep pits and pyramides are present on the surface and more than 30 layers are uncovered whereas at 910K after the removal of the same amount only 2 layers are uncovered. The vacancies created by sputtering in the surface layer nucleate to vacancy islands. These vacancy islands are the basic morphological unit on the surface after sputtering with Ar$^{+}$ ions. Above 700K sputtering temperature the shapes of the islands become almost uniform. A detailed analysis reveals that at these temperatures the resulting vacancy islands can be regarded as two-dimensional (negative) crystals which exhibit an equilibrium shape. It is found that the coincidence of the the onset temperatures of the layer-by-layer removal and of the equilibration of the vacancy islands is due to the same mechanism, determining the two processes, namely a strong increase in the density of the Pt-adatom lattice gas on the vacancy island area. The annealing of a sputtered sample to a temperature higher than the sputtering temperature leads to a significant reduction in the number of islands per unit area. The final island density is shown to be largely independent of the initial one and is determined primarily by the annealing temperature. These observations give evidence for Ostwaldripening. The ripening takes place by an exchange of vacancies between the vacancy islands. A quantitative analysis of the dependence of the observed island density on sputtering temperature and fluence has been made. Differences in the island size distributions of sputtered and of sputtered and then annealed samples are discussed. Insight into the sputtering process itself has been gained from samples sputtered at low temperatures, when most of the thermally activated atomic processes are "frozen" in. Most remarkable is the evidence obtained from the STM topographs for the generation of adatoms by ion bombardment. The observed morphological effects after sputtering the Pt(111) surface are explained on the basis of a simple model for thermally activated atomic processes. By means of this model it is possible to relate specific atomic processes to onset-temperatures of morphological effects. Besides the sputtering effects already known from Ar$^{+}$ ion bombardment, the low energy He$^{+}$ ion bombardment causes strong effects on the surface morphology, resulting from the implantation of the primary ions. It could be confirmed that helium bubbles grow below the surface by the dislocation loop punching mechanism. In the case of low implantation energy, i. e. low bubble depth, the punched out loops move through the crystal towards the surface. At the surface the loops anneal completely by the formation of adatom islands. This dislocation loop annealinginto adatom islands at the surface demonstrates the importance of the attractive elastic interaction between the punched out loops and the surface for the understanding of the "implantation morphology". Iterative loop punching and surface annealing leads to adatom island pyramides on the surface. The formation of microprotrusions on field emitter tips after light ion bombardment is reinterpreted and it is shown that the microprotrusions are due to iterative loop punching and annealing. The evolution of the surface morphology with increasing He$^{+}$ ion fluence is studied in detail: Subatomic height bumps, adatom islands, dislocation networks and finally pit in the surface are formed successively.
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